This invention related to web splicing apparatus. It relates more particularly to apparatus of this type which makes a splice between two webs while minimizing tension upsets in the web. Although the invention has application to splicers used in conjunction with unwind stands as well as those used with rewind stands, we will specifically describe the invention only in the former context.
Constant tension web splicers have been used for many years to splice the trailing end of a running web to the leading end of a ready web so that web can proceed uninterruptedly to a downstream web consuming machine. A splicer typically includes a pair of roll stands for holding web rolls, one of which is running and the other of which is at the ready.
Web from the running roll is guided through a splicing station and into a festoon where a supply of web is maintained to service the downstream web consuming machine when the running roll is slowed or stopped to make a splice. When the running roll is about to expire, it is braked and the leading end of the ready web, already prepared and placed at the splicing station, is adhered to the running web at the splicing station. Then the web from the running roll is severed upstream from the splice and the ready web is accelerated up to line speed with the depleted festoon being refilled with web in the process.
Normally, to minimize tension upsets in the web, the position of the festoon dancer is monitored with respect to a selected reference position to produce an error signal that is indicative of a tension change in the web. This error signal is then used to control the brakes on the unwind stand (or the winding motor on the rewind stand) so that the brakes release or retard the running web as needed to return the dancer to its reference position and thus relieve the tension upset.
Invariably, prior web splicing apparatus maintain constant tension in the web by adjusting the braking torque on the running roll. Consequently the web tension correction is, of necessity, applied through the running roll whose size is constantly changing. Therefore, in order to maintain even reasonably stable festoon control, the overall gain of the control system has to be changed to compensate for the change in roll size. Thus the prior splicing systems require various follower arms, optical sensors or the like to monitor roll size and the ancillary electronics to convert that measurement into the required system gain change, making the prior apparatus unduly complicated and costly.
Further, even in the prior arrangements which do compensate for change in roll size, stable festoon control is not achieved because of irregular characteristics in the usual roll stand brakes due to wear, bearing conditions and the like. These conditions prevent the application of a braking torque to the roll stand in response to a dancer position error signal that when coupled through the unwinding roll results in a web tension change that precisely returns the dancer to its reference position. This problem can be overcome to some extent by using high performance brakes whose characteristics do not vary appreciably with time, wear, excessive heating, etc. However the cost of obtaining such brakes is very high. Moreover, the cost of operating these brakes, as well as other conventional brakes, is quite high because they are always energized to some extent to impart a drag on the running web and a large amount of this energy is dissipated as heat.
Still further, stable festoon control is difficult to achieve with conventional splicers because the festoon dancer invariably has considerable inertia which slows its response to tension upsets. This, in turn, stems from the fact that the dancer has to be able to withstand very strong impacts in the event that it is driven to the stops during a web break and also because the dancer is biased towards its maximum storage position by pneumatic cylinders which contain trapped air and have high starting friction.
Another disadvantage of conventional splicing systems which control web tension by varying the braking torque applied to the running roll is that the entire span of web between the festoon dancer and the running roll is under full tension. As the length of the tensioned web span increases, there is a greater likelihood of there being a weak spot that could be the site of an incipient web break. Therefore it is highly desirable to minimize the length of the web span that is maintained under full tension. Also in actual practice, when large web rolls are braked, an extremely large torque is coupled through the roll core and the roll convolutions adjacent the core that tends to cause tears at those points.
Finally, prior systems of this general type require a relatively long time to effect the splice because of the time delays and inertia inherent in the splicing nips and knives. Nor do they attempt to control the deceleration and acceleration phases of the splicing sequence to apply the least necessary tension to the particular web being spliced.